Implantable sensor for measuring physiologic information

ABSTRACT

An implantable sensor is provided that includes a piezopolymer sensor element including a body having a plurality of layers of a piezopolymer, and an attachment device configured to hold the piezopolymer sensor element in direct contact with at least one of a bodily fluid and bodily tissue such that the piezopolymer sensor element is configured to bend in response to motion of the at least one of bodily fluid and bodily tissue. A pair of electrodes is attached to the piezopolymer sensor element and the electrodes are configured to collect an electrical charge that is generated within the piezopolymer sensor element due to the bending of the piezopolymer sensor element.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending U.S. patent applicationSer. No. 11/750,226, filed May 17, 2007, titled “Implantable Sensor forMeasuring Physiologic Information.”

FIELD OF THE INVENTION

Embodiments of the present invention pertain generally to methods andsystems for measuring cardiac-related physiologic information utilizingan implantable sensor, for example, for the treatment and/or diagnosisof congestive heart failure.

BACKGROUND OF THE INVENTION

In the United States, it is estimated that approximately five millionpeople have congestive heart failure (CHF). During treatment of some CHFpatients, the contractility of the heart may be assessed for diagnosisand/or treatment purposes. In addition to being useful for generallymonitoring the progression of a patient's cardiac disease, the heart'scontractility may be monitored over time to monitor the patient'sresponse to therapy and make any appropriate changes thereto. Forexample, some patients suffering from CHF have an abnormality in theelectrical conducting system of the heart, sometimes referred to as“intraventricular conduction delay” or “bundle branch block” that causesthe left and right ventricles to beat out of phase instead ofsimultaneously. Cardiac resynchronization therapy (CRT), sometimesreferred to as “biventricular pacing”, may be used to re-coordinate thebeating of the left and right ventricles by pacing both ventriclessimultaneously. In contrast to CRT, conventional pacemakers typicallyonly pace the right ventricle. During treatment of CRT patients, thesystolic and diastolic pump properties of one or both of the ventriclesmay be assessed to assist pacing the ventricles and/or monitoring theheart's contractility over time.

Some known methods of assessing the heart's contractility includeintroducing a conventional blood pressure or pressure-volume looptransducer into the ventricle chamber from the femoral artery todetermine the ventricular blood pressure and volume. However, suchtransducers are typically removed after each measurement because thepresence of the transducer and associated components within the leftventricle risk causing a stroke. Accordingly, conventional transducersare generally not implantable within the left ventricle for continuouslymonitoring heart contractility over time. Medical imaging, such as echoimaging or magnetic resonance imaging (MIR), can be used tonon-invasively measure ventricular blood pressure. However, medicalimaging procedures are typically lengthy and expensive, and thereforemay not be suitable for monitoring cardiac contractility over timebecause of the cost and/or inconvenience of the multiple of proceduresto the patient.

Conventional diaphragm-type sensors (force gauges) may be implantablewithin the ventricles or within the pericardial space adjacent theventricles for continuously monitoring contractility over time. However,conventional diaphragm-type sensors may need to be hermetically sealedto operate within the human body, and are typically battery-powered.Conventional diaphragm-type sensors may therefore be bulkier and/or lessreliable than is desired for implantation within or adjacent to theheart. Moreover, the battery may limit the duration for whichconventional diaphragm-type sensors may remain operable within the humanbody without being serviced or replaced.

Sensors fabricated from lead zirconate titnate (PZT), a piezoelectricceramic material, have been contemplated for use as an implantablesensor that measures heart contractility. However, because PZT has arelatively large d₃₃ coefficient, PZT sensors are typically sensitive tohydrostatic pressure and sound waves. Specifically, hydrostatic pressureand/or sound waves may cause the PZT sensor to respond to movement in adirection approximately parallel to a thickness of the PZT sensor. Theresponse caused by such movement may add undesirable noise to themeasurement signal of the PZT sensor that represents the motion of theheart, thereby reducing an overall signal clarity of heart contractilityinformation. The relatively large d33 coefficient of PZT causes PZT tobe sensitive to triboelectric charges generated within the PZT sensor byfriction between the PZT sensor and surfaces with which the PZT sensoris in contact, thereby further reducing signal clarity of the heartcontractility information. Moreover, because PZT contains lead, PZT maynot be biocompatible and therefore not be suitable for implantationwithin the human body.

A need remains for an implantable sensor that is directed to overcomingone or more of the problems set forth above. A need remains for animplantable sensor with improved signal clarity that is biocompatibleand does not require an external power source and/or hermetic sealing.

SUMMARY

In one embodiment, an implantable sensor is provided that includes apiezopolymer sensor element including a body having a plurality oflayers of a piezopolymer, and an attachment device configured to holdthe piezopolymer sensor element in direct contact with at least one of abodily fluid and bodily tissue such that the piezopolymer sensor elementis configured to bend in response to motion of the at least one ofbodily fluid and bodily tissue. A pair of electrodes are attached to thepiezopolymer sensor element and the electrodes are configured to collectan electrical charge that is generated within the piezopolymer sensorelement due to the bending of the piezopolymer sensor element.

Optionally, the piezopolymer sensor element may include polyvinylidenefluoride (PVDF). In some embodiments, the piezopolymer sensor elementmay be a first piezopolymer sensor element, and the implantable sensormay further include a second piezopolymer sensor element held by theattachment device or another attachment device. The first and secondpiezopolymer sensor elements may be held in a desired orientation withrespect to one another such that the second piezopolymer sensor elementbends in a direction arranged approximately perpendicular to the firstpiezopolymer sensor element. In some embodiments, the attachment deviceis configured to hold the piezopolymer sensor element in direct contactwith blood flowing through a ventricle of the heart. The attachmentdevice may be configured, in some embodiments, to hold the piezopolymersensor element in direct contact with an epicardial surface of theheart, an endocardial surface of the heart, and/or a surface of apericardium. A radio-frequency (RF) transmitter may optionally beelectrically connected to the electrodes for transmitting an electricalvoltage output of the electrodes.

In another embodiment, a method is provided for measuring the motion ofa heart. The method includes positioning a sensor element within apericardial space of a pericardium of the heart in contact with at leastone of bodily tissue and bodily fluid such that the sensor element bendsin response to motion of the at least one of bodily tissue and bodilyfluid, measuring an output voltage of electrical charges generated on atleast one surface of the sensor element due to the bending of the sensorelement, and determining a bending moment of the sensor element based onthe measured output voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an implantable sensor system formed in accordancewith an embodiment of the present invention.

FIG. 2 illustrates the implantable sensor system shown in FIG. 1positioned in accordance with an alternative embodiment of the presentinvention.

FIG. 3 illustrates the sensor element of the system shown in FIGS. 1 and2 formed in accordance with an embodiment of the present invention.

FIG. 4 illustrates an exemplary measurement signal generated by theimplantable sensor system shown in FIG. 2 in response to motion of aheart.

FIG. 5 illustrates a sensor element formed in accordance with analternative embodiment of the present invention.

FIG. 6 illustrates a sensor element formed in accordance with anotheralternative embodiment of the present invention.

FIG. 7 illustrates an attachment device of the system shown in FIGS. 1and 2 formed in accordance with an embodiment of the present invention.

FIG. 8 illustrates an attachment device formed in accordance with analternative embodiment of the present invention.

FIG. 9 illustrates an attachment device formed in accordance withanother alternative embodiment of the present invention.

FIG. 10 illustrates an attachment device formed in accordance withanother alternative embodiment of the present invention.

FIG. 11 illustrates an attachment device formed in accordance withanother alternative embodiment of the present invention.

FIG. 12 illustrates an attachment device formed in accordance withanother alternative embodiment of the present invention.

FIG. 13 illustrates an exemplary embodiment of a method for positioninga sensor element.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an implantable sensor system 10 formed in accordancewith an embodiment of the present invention. The sensor system 10 may beused, for example, for generally monitoring the contractility of apatient's heart 12, e.g., monitoring the diastolic and systolic pumpproperties of one or both of the ventricles of the heart. The system 10includes a sensor element 14, an attachment device 16 for positioningthe sensor element in direct contact with bodily tissue and/or bodilyfluid, and a processing unit 18. The sensor element 14 is fabricatedusing a “piezopolymer”, which as used herein is intended to mean apolymer having piezoelectric properties. The sensor element 14 may befabricated from any suitable piezopolymer(s) that enables the sensorelement 14 to function as described herein, such as, but not limited to,polyvinylidene fluoride (PVDF).

As will be described in more detail below, the sensor element 14 bendsin response to motion of the bodily tissue and/or bodily fluid. As thesensor element bends, an electrical charge is generated within a body 20(FIG. 3) of the sensor element 14. A voltage of the electrical charge isproportional to the bending moment of the sensor element 14, which canbe used to determine motion properties of the fluid and/or tissue whosemotion caused the sensor element to bend. In some embodiments, twosensor elements 14 may be held, whether by the same or a differentattachment device 16, in a desired orientation with respect to oneanother such that the two sensor elements bend in directions arrangedapproximately perpendicular to each other (e.g., the sensor elements 614and 714, shown in FIG. 10, and the pair of sensor elements 14 shown inFIG. 12).

In the exemplary embodiments, the sensor element 14 is positioned withinand/or adjacent the heart 12 for obtaining contractility informationabout the heart, e.g., for measuring diastolic and systolic pumpproperties of the heart. However, the sensor system 10 is not limited touse with the heart 12, but rather may be used to measure the motion ofany bodily tissue and/or bodily fluid, such as, but not limited to,joints, the lungs, and/or other muscles within the body besides theheart. When used to measure the contractility of the heart 12, thesensor element 14 may be positioned anywhere within, on, and/or adjacentthe heart that is suitable for measuring motion of the heart itselfand/or blood being pumped through the heart. For example, the sensorelement 14 may be positioned in the pericardial space in direct contactwith an epicardial, or external surface of the heart 12, as is shown inFIG. 1 wherein the sensor element 14 is positioned on an epicardialsurface 22 of a left ventricle 25 of the heart. The sensor element 14may also be positioned in direct contact with an endocardial, orinternal, surface of the heart 12, e.g., an endocardial surface 24 ofthe left ventricle 25.

Positioning the sensor element 14 in direct contact with a surface ofthe heart 12 enables direct measurement of the motion of the surface.Alternatively, the sensor element 14 may not be positioned in directcontact with a surface of the heart 12, but rather may be positioned indirect contact with other bodily tissue that is adjacent to, or indirect contact with, a surface of the heart 12. For example, the sensorelement 14 may be positioned within a pericardial space 13 (FIG. 2) indirect contact with a surface of the pericardium 15 (FIG. 2), e.g., thesurface 17 (FIG. 2) of a serous pericardium 19 (FIG. 2) of thepericardium and/or a surface 21 (FIG. 2) of a fibrous pericardium 23 ofthe pericardium.

The sensor element 14 may also be positioned to indirectly measure themotion of the heart by measuring other properties that relate to, andcan be used to determine, motion. For example, the sensor element 14 canbe positioned within the heart 12 in direct contact with a flow path ofblood through the heart. Contractility information about the heart 12can then be determined based on the motion of blood through the heart.One example of measuring blood flow includes positioning the sensorelement 14 within a ventricle (e.g., the left ventricle 25) of the heart12 in direct contact with blood flowing through the ventricle to measureblood flow therethrough. The sensor element 14 may also be positionedon, adjacent, and/or within vessels leading into and out of the heart 12(e.g., the aorta) to obtain contractility information about the heart.

The attachment device 16 may include any suitable structures, elements,components, configurations, arrangements, and/or geometries thatsecurely positions and holds the sensor element 14 in the positions(e.g., location and/or orientation) described and/or illustrated herein.The attachment device 16 may form a portion of a lead, as shown in FIG.1 with respect to the lead 26. Alternatively, the attachment device 16may not form a portion of, and/or may not be connected to, a lead.Various examples of attachment devices are described below with respectto FIGS. 8-11.

The position of the sensor element 14 may be selected anywhere within,on, and/or adjacent the heart 12 to determine contractility informationof the heart overall and/or at specific locations adjacent the sensorelement 14. In some embodiments, to provide more comprehensivecontractility information, a plurality of sensor elements 14 (whetherpart of the same lead or whether connected to the same RF transmitter)may be positioned at different locations within, on, and/or adjacent theheart 12 and/or at different orientations with respect to other sensorelements. For example, because motion of the heart 12 is generally notlinear, it may be desirable to position a pair of sensor elements 14 asoriented approximately perpendicular to each other such that the pair ofsensor elements each bend in a direction arranged approximatelyperpendicularly to each other (e.g., for measuring both short and longaxis motion of heart 12).

Generally, the contractility of the heart is assessed using the sensorsystem 10 for diagnosis and/or treatment of the patient. In addition tobeing useful for generally monitoring the progression of a patient'scardiac disease, contractility of the heart may be monitored over timeto monitor the patient's response to therapy and make any appropriatechanges thereto. For example, the contractility information obtained bythe sensor system 10 may provide a physician with information as towhether the hemodynamic functioning of a patient has improved, how scarformation is progressing, the status of local or global heart failure,and/or the like. The contractility information may also be used incombination with other systems to control functions of such othersystems. For example, the contractility information may be used by, butis not limited to being used by, another system (not shown), such as,but not limited to, a pulse generator, a pacemaker, an implantablecardioverter defibrillator, a defibrillator, a therapy delivery modulethat paces and/or provides electrical stimulation to the heart 12,and/or the like to, for example, control an appropriate pacing scheme ordefibrillation event. One example of a specific use for the sensorsystem 10 is for determining the least contraction site of the heart 12where a pacing electrode (not shown) can be positioned for optimalpacing. External systems (not shown), such as, but not limited to, anexternal health monitoring system at a treatment facility and/or thepatient's home may also make use of the contractility information fortreatment and/or diagnosis purposes.

Voltage measurement signals from the sensor element 14 are provided tothe processing unit 18 via an electrical connection therebetween. Thesensor element 14 may be electrically connected to the processing unit18 through a lead 26, as is shown in FIG. 1. Optionally, the sensorelement 14 may be incorporated into the lead of another system (notshown), such as, but not limited to a pulse generator, a pacemaker, animplantable cardioverter defibrillator, a defibrillator, a therapydelivery module that paces and/or provides electrical stimulation to theheart 12, and/or the like. Additionally or alternatively to a lead, thesensor element 14 may be electrically connected to the processing unit18 using a wireless connection, such as, but not limited to, using an RFtransmitter 36 (FIG. 2) electrically connected to the sensor element 14.

The processing unit 18 may process the voltage measurement signalsreceived from the sensor element 14 to determine the bending momentrepresented by each of the signals. The processing unit 18 may furtherprocess the bending moments to determine contractility information ofthe heart 12 or portions thereof, e.g., the systolic and/or diastolicpump properties of one or both of the ventricles. The processing unit 18includes a memory (not shown) for storing the voltage measurementsignals received from the sensor element 14, as well as for storing anydetermined bending moments, any determined contractility information,and/or other information relevant to treatment and/or diagnosis of thepatient.

The processing unit 18 communicates with an external system 28, such as,but not limited to, an external health monitoring system at a treatmentfacility and/or the patient's home, and/or a laptop, handheld, ordesktop computer at the treatment facility. The processing unit 18 maycommunicate any determined contractility and/or other relevantinformation to the external system 28 for use by the external system 28and/or a physician in diagnosing, treating, and/or monitoring thepatient. Additionally or alternatively, the external system 28 maydetermine contractility information of the heart by processing voltagemeasurement signals, any determined bending moments, and/or otherrelevant information received from the processing unit 18. Theprocessing unit 18 may communicate with the external system 28 using awireless connection (as shown in FIG. 1) and/or wired connection.

The processing unit 18 may also cooperate with other systems implantedwithin the patient's body to make use of the contractility informationobtained by the sensor system 10, as is described above. For example,the processing unit 18 may optionally communicate with, constitute theprocessing unit of, or be incorporated into another system (not shown)implanted within the patient's body, such as, but not limited to a pulsegenerator, a pacemaker, an implantable cardioverter defibrillator, adefibrillator, a therapy delivery module that paces and/or provideselectrical stimulation to the heart 12, and/or the like.

The processing unit 18 may be implanted at any suitable location withinthe body that enables it to function as described herein, such as, butnot limited to, in the abdomen 30 (as shown in FIG. 1) or a pectoralmuscle 32. Alternatively, the processing unit 18 may be positionedexternally to the patient's body. For example, the processing unit 18may be worn externally on the patient's hip 34 or another body portion,or may remain at a treatment facility and be connected (e.g., via a leador wirelessly) to the sensor element 14 when the patient comes to thefacility for treatment.

FIG. 2 illustrates the sensor system 10 positioned in accordance with analternative embodiment of the present invention. Specifically, as shownin FIG. 2, the sensor element 14 is positioned within the pericardialspace 13 in direct contact with the surface 17 of the serous pericardium19 of the pericardium 15, as opposed to the epicardial surface 22 asshown in FIG. 1. Further, the sensor element 14 is electricallyconnected to the processing unit 18 using a radio-frequency (RF)transmitter 36 electrically connected to the sensor element 14, asopposed to the lead 26 (FIG. 1).

FIG. 3 illustrates a sensor element 14 in more detail. The sensorelement 14 includes the body 20 extending between a pair of opposite endportions 38 and 40. In the exemplary embodiment of FIG. 3, the sensorbody 20 includes a plurality of layers 41 of piezopolymer that aresandwiched together. Alternatively, the sensor body 20 includes only onelayer of piezopolymer. A surface 35 of one of the layers 41 a of thebody 20 may optionally be at least partially coated with one or morelayers of any suitable electrically conductive material 37, such as, butnot limited to, silver and/or another electrical conductor. In theexemplary embodiment, the body 20 includes two layers of theelectrically conductive material 37 on the surface 35. In someembodiments, a surface 43 of another of the layers 41 b may also be atleast partially coated with one or more layers of any suitableelectrically conductive material.

A pair of electrodes 42 and 44 are attached to the proximate end portion40 of the body 20 on an exposed surface 39 of the outermost layer of theelectrically conductive material 37. As described above, the sensor body20 is fabricated from a piezopolymer, such as, but not limited to, PVDF.In the exemplary embodiment of FIG. 3, the sensor body 20 issubstantially longer in the longitudinal direction along length L, ascompared to the width W that extends in the transverse direction. Forexample, the sensor body 20 may have a length L that is 3 or more timesthe width W. As a further option, the length L may be ten or more timesthe width W. The piezopolymer material of the sensor body 20 is agenerally flexible material that enables the sensor element 14 to bendwhen in contact with bodily tissue and/or fluid that is in motion.

The plurality of layers 41 may facilitate increasing a measurementsensitivity of the sensor body 20 as compared with only one layer. Forexample, each layer 41 may generate the same voltage output for the sameamount of motion such that the overall voltage output is N times larger(where N is the number of layers 41). The layers 41 may be attachedtogether using any suitable method, process, structure, means, and/orthe like, such as, but not limited to, using heat, compression, and/oradhesive, such as, but not limited to, and electrically conductiveadhesive, such as, but not limited to, silver epoxy. Although fourlayers 41 are shown, the sensor body 20 may include any number of layers41.

Each layer 41 of the sensor body 20 may be fabricated, for example, byrepeatedly pulling the piezopolymer material along the molecular chaindirection of the material, whether the layers 41 are pulled together orseparately. During pulling thereof, each layer 41 is electricallypolarized. After such a fabrication process, the sensor body 20generates the strongest signals along the molecular chain direction,e.g., along the longitudinal axis denoted by the length L of the sensorbody 20. As discussed above, as the sensor body 20 bends along thelongitudinal axis or the length L, an electrical charge is generatedwithin the body 20, which collects at the electrodes 42 and 44. Anoutput voltage of the electrical charge across the pair of electrodes 42and 44 is proportional to the bending moment and the rate of bending ofthe sensor element 14. For example, the charge collected at theelectrodes 42 and 44 may increase linearly in proportion to the amountthat the sensor element 14 is bent along the longitudinal axis.

Optionally, the charge at the electrodes 42 and 44 may increasenon-linearly, but in a known relation, in proportion to the amount thatthe sensor element 14 is bent. For example, the non-linear proportionmay be exponential, a second or third order polynomial, and the like.The charge at the electrodes 42 and 44 is measured as a voltagepotential and used to determine motion properties of the fluid and/ortissue whose motion caused the sensor element 14 to bend. The electrodes42 and 44 are electrical connected to either the lead 26 (FIG. 1) or theRF transmitter 36 (FIG. 2), for example using leads 46 and 48, toprovide the electrical connection between the sensor element 14 and theprocessing unit 18 (FIG. 1).

The sensor body 20 may have any suitable size, shape, and/or number oflayers 41 that enables the sensor element 14 to function as describedherein. The output voltage across the pair of electrodes 42 and 44 isproportional to the amount of surface area of the sensor body 20 that isstressed in the direction of the length L, such that the larger thesurface area, the greater the charge, voltage, and signal strength. Thesize, shape, and/or number of layers 41 of the sensor body 20 may beselected to provide a desired voltage or signal strength. Although thesensor body 20 may have many other suitable shapes, in the exemplaryembodiment shown in FIG. 3, the sensor body 20 is an elongated beamhaving a generally rectangular shape and cross section. Other suitableshapes of the sensor body include, but are not limited to, elliptical,square and/or tubular shapes. A shape that is elongate about themolecular chain direction, such as, but not limited to the elongate beam20 shown in FIG. 3, may facilitate an increased surface area that isstressed during bending, and therefore facilitate increased signalstrength. Exemplary sizes for an elongate beam that may be suitable forimplantation adjacent, on, and/or within the heart 12 (FIGS. 1 and 2)include, but are not limited to, a length L of between approximately 3mm and approximately 11 mm, a width W of between approximately 1 mm andapproximately 5 mm, and a thickness T of between approximately 5 μm andapproximately 100 μm. By way of example, a sensor body having a lengthof approximately 10 mm, a width of approximately 4 mm, and a thicknessof approximately 25 μm may generate in the order of hundreds ofmillivolts to ones of Volts. The net voltage across the pair ofelectrodes 42 and 44 generated by the bending of the body 20 can berepresented by the piezocoefficient d₃₁.

As the piezopolymer material of the sensor body 20 generates signalsprimarily along the molecular chain direction (along the length L), thesensor element 14 has a relatively small value (e.g., approximatelyzero) of the piezocoefficient d₃₃, which represents the ratio of thepressure to the generated voltage of the piezopolymer material of thesensor body 20. The smaller d₃₃ coefficient of the piezopolymer materialof the sensor body 20 reduces an amount of noise signals within themeasurement signals of the sensor element 14, for example caused byhydrostatic pressure. Accordingly, use of the piezopolymer material ofthe sensor body 20 may result in higher signal clarity when the sensorbody is used to measure motion. Although the piezopolymer material ofthe sensor body 20 has a relatively small d₃₁ value, the piezopolymermaterial of the sensor body is generally flexible. The piezopolymermaterial of the sensor body 20 will therefore readily bend, which maygenerate a signal strength representing bending along the length L ofthe sensor body that is quite large. By way of example, the piezopolymermaterial of the sensor body 20 may produce a signal strength thatrepresents bending along the length L of the sensor body that is in theorder of ones of millivolts to tens of Volts.

Because the measurement signals of the sensor element 14 are generatedby bending of the sensor body 20, the sensor element does not require anexternal power source for measuring motion. Moreover, the piezopolymermaterial of the sensor body 20 is generally biocompatible, and thereforeis generally suitable for implantation within the human body and may notrequire hermetic sealing.

The electrodes 42 and 44 may each have any suitable position on thesensor body 20 that enables them to function as described herein.Generally, as long as the electrodes 42 and 44 are spaced apart from oneanother such that the electrodes 42 and 44 are not electricallyconnected together, the position of the electrodes 42 and 44 may notaffect the signal strength or more specifically the amount of voltagepotential across the electrodes 42 and 44. In the exemplary embodimentof FIG. 3, the electrodes 42 and 44 are positioned adjacent each otheron the same side portion 50 and the same end portion 40 of the sensorbody 20.

The electrodes 42 and 44 may be fabricated using any suitableelectrically conductive material(s) that enables them to function asdescribed herein, such as, but not limited to silver, aluminum, gold,copper, nickel, palladium, platinum, rhodium, rhenium, tin, othermetallic conductors, and/or the like. In the exemplary embodiments, eachof the electrodes 42 and 44 is formed by a layer of silver that isprint-coated on the sensor body 20. In the exemplary embodiment, theelectrodes 42 and 44, and/or any connection between the electrodes 42and 44 and the lead 26 and/or the transmitter 36, are be coated with anysuitable insulating material (not shown in FIG. 3).

FIG. 4 illustrates an exemplary measured signal 54 generated by thesensor element 14 in response to motion. In the embodiment representedin FIG. 4, the sensor element 14 has been implanted and positionedwithin the pericardial space 13 (FIG. 2) adjacent the left ventricle 25in direct contact with the surface 17 of the serous pericardium 19 ofthe pericardium 15. The sensor element 14 has been orientated such thatthe length L of the sensor body 20 is approximately parallel to theshort axis of the heart (e.g., as shown in FIG. 2). The sensor element14 is thus used to measure the heart motion along the short axis. Themeasured signal 54 represents the motion of the left ventricle overtime. Also shown are signals 56, 58, and 60, respectively, representingan electrocardiogram (ECG), the aortic pressure (AoP), and thederivative of the blood pressure (dp/dt) of the heart over the same timeperiod.

FIGS. 5 and 6 illustrate sensor elements 114 and 214, respectively,having exemplary alternative shapes and exemplary alternative electrodepositions than the previously described and/or illustrated sensorelement 14. Specifically, FIG. 5 illustrates the sensor element 114having electrodes 142 and 144 attached on opposite side portions 150 and152 of a body 120 of the sensor element 114. The sensor body 120 has agenerally square shape in contrast to the elongate beam of the sensorbody 20 (FIG. 3). Although shown as having only one layer ofpiezopolymer, the sensor body 120 may alternatively include two or morelayers of piezopolymer. FIG. 6 illustrates the sensor element 214 havingthe electrodes 242 and 244 attached on opposite ends 238 and 240 of abody 220 of the sensor element 214. The sensor body 220 has a generallyelongate tubular shape in contrast to the elongate beam of the sensorbody 20 (FIG. 3). Although shown as cylindrical, the tubular shape ofthe sensor body 220 may alternatively be a tubular shape that is notgenerally cylindrical (e.g., a square tube). Although shown as havingonly one layer of piezopolymer, the sensor body 220 may alternativelyinclude two or more layers of piezopolymer.

FIG. 7 illustrates an attachment device 316 formed in accordance with anembodiment of the present invention. The attachment device 316 forms theend portion 362 of a lead (e.g., the lead 26 shown in FIG. 1).Specifically, the attachment device 316 includes a loop-shaped portion364 that is formed by a cable 366 of the lead. The sensor element 14 isheld within a space 368 bounded by the loop-shaped portion 364. Thesensor element 14 and the loop-shaped portion 364 generally lie in aplane P₁.

The sensor element 14 may be held within the loop-shaped portion 364using any suitable structure, method, configuration, arrangement,orientation, and/or means that enable the sensor element 14 to functionas described herein. In the exemplary embodiment of FIG. 7, the sensorelement 14 is held within the loop-shaped portion 364 using supports370. Although the supports 370 may have any suitable configurationand/or arrangement, in the exemplary embodiment the supports attached tothe lead cable 366 and extending across the space 368 bounded by theloop-shaped portion 364 are attached to the sensor body 20 at each ofthe ends 38 and 40 thereof.

The supports 370 may be fabricated from any suitable material(s) thatare capable of securely holding the sensor body 20 in the position shownin FIG. 7, such as, but not limited to, silicone, polyurethane,Hemoflex®, and/or an adhesive, such as, but not limited to, a medicalgrade adhesive, such as, but not limited to, Med A (commerciallyavailable from Dow Chemical of Midlands, Mich.). An insulating material372 encapsulates the electrodes 42 and 44 and the respective leads 46and 48 of the electrodes 42 and 44 to insulate the electricalconnections of the sensor element 14. The cable 366 may be fabricatedfrom any suitable material(s) that enable the attachment device 316 tofunction as described herein. Optionally, the lead is a lead of anothersystem (not shown), such as, but not limited to a pulse generator, apacemaker, an implantable cardioverter defibrillator, a defibrillator, atherapy delivery module that paces and/or provides electricalstimulation to the heart 12, and/or the like.

The attachment device 316 may be positioned adjacent, on, and/or withinthe heart 12 (FIG. 1) (or other bodily tissue and/or fluid) bypositioning the end portion 362 of the lead at the desired location ofthe heart 12 such that the sensor element 14 is positioned at thedesired location and in the desired orientation. Because the loop-shapedportion 364 is connected to the lead cable 366, the sensing element 14will remain at the desired location and in the desired orientation aslong as the lead cable 366 is not moved. Frictional forces between thesensor body 20 and/or the lead cable 366 and adjacent surfaces withwhich the sensor body and/or the lead cable are in contact may alsofacilitate securely holding the sensor element 14 at the desiredlocation and in the desired orientation. For example, frictional forcesbetween the sensor body and a surface of the heart 12 or a surface ofthe pericardium 15 (FIG. 2) may facilitate holding the sensor element 14in direct contact with the surface. When the sensor element isorientated to be within a flow path of blood within the heart 12,frictional forces between lead cables 366 and surfaces defining theblood flow path may facilitate securely holding the sensor elementwithin the flow path.

FIG. 8 illustrates an attachment device 416 formed in accordance with anembodiment of the present invention. The attachment device 416 includesa diamond-shaped portion 464 that is formed by a cable 466. A sensorelement 414 is held within a space 468 bounded by the diamond-shapedportion 464. The sensor element 414 and the diamond-shaped portion 464generally lie in a plane P₂. However, one or more cable portions 474 ofthe diamond-shaped portion 464 may be bowed outwardly such that theportions 474 extend outwardly away from the plane P₂. Although twoportions 474 are shown as extending outwardly in generally the samedirection, any number of portions 474 may be bowed outwardly, with eachportion 474 extending outwardly in any direction relative to the planeP₂ and/or the other portions. For example, two portions 474 may extendoutwardly from the plane P₂ in generally opposite directions.

The sensor element 414 may be held within the diamond-shaped portion 464using any suitable structure, configuration, arrangement, orientation,method, and/or means that enable the sensor element 414 to function asdescribed herein. In the exemplary embodiment of FIG. 8, the sensorelement 414 is held within the diamond-shaped portion 464 using supports470. Although the supports 470 may have any suitable configurationand/or arrangement, in the exemplary embodiment a pair of supports 470are attached to opposite ends 438 and 440 of the sensor body 420, whereeach support 470 extends away from the sensor body 420 to attach to anopposite corner 476 of the diamond-shaped portion 464.

The attachment device 416 may be positioned adjacent, on, and/or withinthe heart 12 (FIG. 1) (or other bodily tissue and/or fluid) bypositioning the diamond-shaped portion 464 at the desired location ofthe heart 12 such that the sensor element 414 is positioned at thedesired location and in the desired orientation. The bowed portions 474facilitate securely holding the sensor element 414 at the desiredlocation and in the desired orientation by providing spring forceagainst adjacent surfaces. For example, when the sensor element 414 andthe attachment device 416 are positioned within the pericardial space,the bowed portions 474 may exert a spring force against the epicardialsurface 22 (FIG. 1) of the heart 12 and/or against a surface of thepericardium 15 (FIG. 2). Frictional forces between the sensor body 420and/or the cable 466 and adjacent surfaces with which the sensor body420 and/or the cable 466 are in contact may also facilitate securelyholding the sensor element 414 at the desired location and in thedesired orientation.

FIG. 9 illustrates an attachment device 516 formed in accordance with anembodiment of the present invention. The attachment device 516 is theend portion 562 of a lead (e.g., the lead 26 shown in FIG. 1).Specifically, the attachment device 516 includes a cable 566 of thelead. The sensor body 520 is received on an exterior surface 578 of thecable 566 such that the sensor body 520 may completely surround aportion of the cable 566. Although the cable 566 may have any suitableshape, and the sensor body 520 may have any suitable shape, in theexemplary embodiment shown in FIG. 9 the cable 566 and the sensor body520 each include a generally cylindrical shape. Although the body 520 ofthe sensor element 514 has a shape that is complimentary with the cable566, alternatively the sensor body 520 and the cable 566 may havedifferent shapes.

FIG. 10 illustrates an attachment device 616 formed in accordance withan embodiment of the present invention. The attachment device 616 issimilar to the attachment device 516 shown in FIG. 9 except that thesensor element 614 only partially surrounds the cable 666. Moreover, thecable 666 includes another sensor element 714 that is orientatedapproximately perpendicular to the sensor element 614 (e.g., formeasuring motion of both the short axis and the long axis of the heart12). Although the sensor elements 614 and 714 are shown as being held bythe same attachment device 616, the sensor elements 614 and 714 mayalternatively be held by different attachment devices.

FIG. 11 illustrates an attachment device 816 formed in accordance withan embodiment of the present invention. The attachment device 816includes a patch 880 having a body 882 configured to be attached tobodily tissue. The sensor element 814 may be held by the patch 880 usingany suitable structure, method, configuration, orientation, arrangement,and/or means. For example, the sensor element 814 may be attached to thepatch body 882 using an adhesive, may be attached to the patch body 882by molding, crimping, welding, and/or suturing, and/or the sensorelement 814 may be embedded between two layers of the patch body.

The patch 880 may include one or a plurality of layers, whether suchlayers are made from the same or different materials. The patch 880 maybe fabricated from any suitable material(s), such as, but not limitedto, polyester, polypropylene, polytetrafluoroethylene, polyurethane,silicone, silicone rubber, and/or the like. The type of materialselected may depend on factors such as the materials softness,stiffness, pliability, flexibility, useful life, biocompatibility,adhesiveness, ability to allow tissue ingrowth, and/or othercharacteristics.

The attachment device 816 may be positioned adjacent, on, and/or withinthe heart 12 (FIGS. 1 and 2) (or other bodily tissue and/or fluid) bypositioning the patch 880 at the desired location of the heart 12 suchthat the sensor element 814 is positioned at the desired location and inthe desired orientation. The patch 880 may include a plurality of pores884 extending within the body 882 to increase the adherence andcompliance of the patch 880 to a surface of the heart 12 by allowingtissue to grow into or through the pores 884. The tissue ingrowth helpsstabilize the patch 880 to the surface of the heart 12. The pores 884are sufficiently sized and shaped to allow an adequate amount of tissueingrowth to enhance the adherence of the patch 880 to the surface of theheart 12. Optionally, the pores 884 may be cylindrical; however, theshape of each of the pores 884 is not limited to a cylindrical shape.Optionally, the pores 884 may have angled walls. In addition oralternative to the pores 884, frictional forces between the sensor body820 and/or the patch body 882 and adjacent surfaces with which thesensor body 820 and/or the patch body 882 are in contact may facilitatesecurely holding the sensor element 814 at the desired location and inthe desired orientation. Another additional or alternative exampleincludes suturing the patch body 882 to a surface of the heart 12 (orother bodily tissue and/or fluid).

FIG. 12 illustrates an attachment device 916 formed in accordance withan embodiment of the present invention. The attachment device 916includes a loop-shaped portion 964 that is formed by a cable 966. A pairof the sensor elements 14 are held within a space 968 bounded by theloop-shaped portion 964. The sensor elements 14 are held by theattachment device 916 such that the sensor elements are orientatedapproximately perpendicular to each other (e.g., for measuring motion ofboth the short axis and the long axis of the heart 12 (FIGS. 1 and 2)).Although the pair of sensor elements 14 are shown as being held by thesame attachment device 916, the pair of sensor elements 14 mayalternatively be held by different attachment devices.

The sensor elements described and/or illustrated herein may be placedinto the positions described and/or illustrated herein using anysuitable method, structure, and/or means. For example, FIG. 13illustrates an exemplary embodiment of a method 1000 for positioning asensor element. When positioning a sensor element within the pericardialspace (e.g., positioning the sensor element 14 (FIG. 2) within thepericardial space 13 in direct contact with the surface 17 of the serouspericardium 19 of the pericardium 15) a needle (not shown, e.g., a Touhyneedle) may be used to puncture 1002 the pericardium 15 (e.g., at thesubxiphoid) and gain access to the pericardial space 13. A guide wire(not shown) is placed 1004 through a central lumen of the needle intothe pericardial space 13. The needle is then withdrawn 1006 and anintroducer (not shown) is placed 1008 over the guide wire into thepericardial space 13. The guide wire is then pulled out 1010 and theattachment device, including the sensor element and any leads associatedwith the attachment device and/or the sensor element, is then forced1012 through the introducer into the pericardial space 13. A stylet (notshown) can then be used to manipulate 1014 the position of theattachment device and thereby the sensor element.

The embodiments described and/or illustrated herein provide animplantable piezoelectric sensor with improved signal clarity over atleast some known piezoelectric sensors. The embodiments described and/orillustrated herein also provide an implantable sensor that isbiocompatible and does not require an external power source and/orhermetic sealing.

Exemplary embodiments are described and/or illustrated herein in detail.The embodiments are not limited to the specific embodiments describedherein, but rather, components and/or steps of each embodiment may beutilized independently and separately from other components and/or stepsdescribed herein. Each component, and/or each step of one embodiment,can also be used in combination with other components and/or steps ofother embodiments. For example, although specific sensor elements aredescribed and/or illustrated with specific attachment devices, eachdescribed and/or illustrated sensor element may be used with any of thedescribed and/or illustrated attachment devices as is appropriate.

When introducing elements/components/etc. described and/or illustratedherein, the articles “a”, “an”, “the”, “said”, and “at least one” areintended to mean that there are one or more of theelement(s)/component(s)/etc. The terms “comprising”, “including” and“having” are intended to be inclusive and mean that there may beadditional element(s)/component(s)/etc. other than the listedelement(s)/component(s)/etc. Moreover, the terms “first,” “second,” and“third,” etc. in the claims are used merely as labels, and are notintended to impose numerical requirements on their objects. Further, thelimitations of the following claims are not written inmeans—plus-function format and are not intended to be interpreted basedon 35 U.S.C. §112, sixth paragraph, unless and until such claimlimitations expressly use the phrase “means for” followed by a statementof function void of further structure.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims.

1. An implantable lead comprising: a first piezopolymer sensor element;a second piezopolymer sensor element; and an attachment deviceconfigured to hold the first piezopolymer sensor element and the secondpiezopolymer sensor element in direct contact with at least one of abodily fluid and bodily tissue such that the piezopolymer sensorelements bend in response to motion of the at least one of bodily fluidand bodily tissue; wherein the first piezopolymer sensor element and thesecond piezopolymer sensor element are held in approximatelyperpendicular orientation with respect to one another.
 2. The lead ofclaim 1 wherein the first and second sensor elements comprisepolyvinylidene fluoride (PVDF).
 3. The lead of claim 1 wherein theattachment device comprises a cable defining an at least partiallyloop-shaped portion having an open space bounded thereby; and the firstand second sensor elements are held within the open space.
 4. The leadof claim 3 wherein the loop-shaped portion defines an at least partiallydiamond-shaped portion.
 5. The lead of claim 4 wherein thediamond-shaped portion comprises one or more cable portions that lie ina common plane with the sensor element and one or more cable portionsthat bow outwardly from the common plane.
 6. The lead of claim 3 furthercomprising a plurality of supports extending from the cable across theopen space, wherein the supports attach to the first and second sensorelements to hold the sensor elements in place within the open space. 7.The lead of claim 1 wherein the attachment device comprises a generallycylindrical cable, and the first and second sensor elements are on anexterior surface of the cable such that the first and second sensorelements at least partially surrounds a portion of the cable.
 8. Thelead of claim 1 wherein the attachment device comprises a patch having abody portion configured to be attached to a surface of the heart.
 9. Thelead of claim 1 wherein the attachment device is configured forendocardial placement.
 10. The lead of claim 1 wherein the attachmentdevice is configured for pericardial placement.
 11. The lead of claim 1further comprising two pairs of electrodes, one pair attached to each ofthe first and second sensor elements, the electrodes configured tocollect an electrical charge generated within the sensor elements due tothe bending of the sensor elements.
 12. The lead of claim 1 wherein thefirst and second sensor elements and the attachment device lie in acommon plane.
 13. A method for measuring the motion of a heart, saidmethod comprising: positioning an attachment device with a sensorelement within a pericardial space of a heart such that the sensorelement is in direct contact with a surface of at least one of a serouspericardium of the pericardium and a fibrous pericardium of thepericardium such that the sensor element bends in response to motion ofthe at least one of a serous pericardium of the pericardium and afibrous pericardium of the pericardium; measuring an output voltage ofelectrical charges generated on at least one surface of the sensorelement due to the bending of the sensor element; and determining abending moment of the sensor element based on the measured outputvoltage.
 14. The method of claim 13 wherein positioning the sensorelement comprises positioning first and second sensor elements incontact with the at least one of a serous pericardium of the pericardiumand a fibrous pericardium of the pericardium and in an approximatelyperpendicular orientation with respect to one another.
 15. The methodaccording to claim 13 wherein positioning the sensor element comprisesintroducing the sensor element into the pericardial space of a heartthrough the subxiphoid.
 16. The method according to claim 13 whereinpositioning the sensor element comprises incorporating the sensorelement into a lead of at least one of a pulse generator, a pacemaker,an implantable cardioverter defibrillator, a defibrillator, and/or atherapy delivery module that at least one of paces and provideselectrical stimulation to the heart.
 17. The method according to claim13 wherein positioning a sensor element comprises positioning apiezopolymer sensor element.
 18. The method according to claim 13further comprising determining information relating to the motion of theat least one of a serous pericardium of the pericardium and a fibrouspericardium of the pericardium based on a determined bending moment.